1. Field of the Invention
The present invention relates, inter alia, to a hybrid between at least one carbohydrate-binding module (“CBM”) and at least the catalytic module (CM) of a glucoamylase. The invention also relates to the use of the hybrid enzyme in a starch process in which granular starch is degraded into sugars, e.g., a syrup, or which may be used as nutrient for yeasts in the production of a fermentation product, such as especially ethanol.
2. Description of Related Art
A large number of processes have been described for converting starch to starch hydrolysates, such as maltose, glucose or specialty syrups, either for use as sweeteners or as precursors for other saccharides such as fructose. Glucose may also be fermented to ethanol or other fermentation products.
Starch is a high molecular-weight polymer consisting of chains of glucose units. It usually consists of about 80% amylopectin and 20% amylose. Amylopectin is a branched polysaccharide in which linear chains of alpha-1,4 D-glucose residues are joined by alpha-1,6 glucosidic linkages.
Amylose is a linear polysaccharide built up of D-glucopyranose units linked together by alpha-1,4 glucosidic linkages. In the case of converting starch into a soluble starch hydrolysate, the starch is depolymerized. The conventional depolymerization process consists of a gelatinization step and two consecutive process steps, namely a liquefaction process and a saccharification process.
Granular starch consists of microscopic granules, which are insoluble in water at room temperature. When an aqueous starch slurry is heated, the granules swell and eventually burst, dispersing the starch molecules into the solution. During this “gelatinization” process there is a dramatic increase in viscosity. As the solids level is 30–40% in a typical industrial process, the starch has to be thinned or “liquefied” so that it can be handled. This reduction in viscosity is today mostly obtained by enzymatic degradation. During the liquefaction step, the long-chained starch is degraded into smaller branched and linear units (maltodextrins) by an alpha-amylase. The liquefaction process is typically carried out at about 105–110° C. for about 5 to 10 minutes followed by about 1–2 hours at about 95° C. The temperature is then lowered to 60° C., a glucoamylase or a beta-amylase and optionally a debranching enzyme, such as an isoamylase or a pullulanase, are added and the saccharification process proceeds for about 24 to 72 hours.
Conventional starch conversion processes are very energy consuming due to the different requirements in terms of temperature during the various steps. It is thus desirable to be able to select and/or design enzymes used in the process so that the overall process can be performed without having to gelatinize the starch.